Particle generators, such as neutron generators and gamma ray generators, are often used together with particle detectors, such as neutron detectors and gamma ray detectors, to log a subsurface formation. For example, by sending particles into a formation, detecting the resulting particles that either bounce back or are generated as a result of the sent particles striking atoms in the formation, and analyzing those detections, useful information about the formation can be discerned and the formation can thus be logged.
A typical particle detector includes a scintillation detector package that is optically coupled to a photomultiplier tube (PMT). The scintillation detector package includes a scintillator crystal that scintillates when struck by particles to which it is attuned (i.e. neutrons, gamma rays). As particles strike the scintillator crystal and cause it to emit light pulses, the PMT detects the light pulses and converts them from the optical domain to the electrical domain. Processing circuitry may then be used to analyze the converted light pulses.
A detector used in a downhole application to log a formation may be exposed to both thermal and mechanical stresses. Unfortunately, mechanical stresses can cause a scintillator crystal to scintillate via triboluminescence, an optical phenomenon in which light is generated when a material is pulled apart, ripped, scratched, crushed, or rubbed, through the breaking of chemical bonds in the material. It thus follows that if the scintillator crystal were to emit a light due to mechanical stress, the PMT might detect and convert that “false” light pulse.
This is an undesirable situation, as the accuracy and resolution of the resulting log could be degraded by the false light pulses. Current approaches at reducing the degradation to a log due to false light pulses revolve around trying to better isolate the scintillation crystal from mechanical stress and shock. While these approaches may indeed be successful in reducing the number of false light pulses emitted by the scintillation crystal, eliminating a majority of false light pulses may be extremely difficult. In addition, these approaches may increase the cost or reduce the size of a detector by an undesirable amount.
As such, new ways of reducing the degradation to a log due to false light pulses emitted by a scintillator crystal is desired.